1. Field of the Invention
The present invention is directed to refractory tube blocks which protect metallic waterwall tubes from hot and highly corrosive furnace gases, while at the same time maintaining good heat conductivity.
2. Background Information
Refractory tiles have long been used for the protection of boiler walls in waste incinerators and other heat exchanger applications. The primary function of the tiles has been to shield the steel alloy boiler walls, which typically include waterwall tubes and membranes or webs disposed therebetween, from the temperature, erosion, and corrosion by acid vapor attack associated with boiler operation. These conditions are generated by the combustion process occurring within the boiler. For example, municipal solid waste (MSW) facilities incinerate trash and garbage in furnaces at temperatures of up to about 1400.degree. C. In order to recover the valuable energy produced in these MSW plants, water is passed through metallic waterwall tubes adjacent to the furnace and converted to steam by the high temperatures. The steam produced in the tube assembly is then used to power a turbine-driven electric generator. However, the MSW plant also produces gaseous products which, if allowed to contact the metal wall, would chemically attack the walls. The purpose of the refractory tiles has been to prevent direct attack of the walls by gaseous products and still allow the tubes to be sufficiently heated to efficiently generate steam. The primary purpose of the tiles is thus to extend tube wall lifetime expectancy.
Refractory tiles for boiler tube protection have traditionally been fabricated from a material such as silicon carbide (SiC). These tiles typically are provided with a substantially planar face with a contoured back surface sized and shaped to match the contour of the tube wall. Such tiles generally have been fabricated as a variant of one of three configurations, namely bolted tiles; hanging tiles, such as disclosed in U.S. Pat. No. 4,768,447 to Roumeguere; and modified hanging tiles also known as slotted or T-slotted tiles such as disclosed in U.S. Pat. No. 5,243,801 to Aiken, et al. and in WO 97/09577 to Zampell Advanced Refractory Technologies, Inc. The U.S. Pat. No. 4,768,447, U.S. Pat. No. 5,243,801 and WO 97/09577 references are fully incorporated by reference herein.
As shown in FIGS. 1 and 2, bolted tiles 10 are generally provided with a square or rectangular face 12 adapted for orientation towards an interior of the boiler. A hole penetrates through the thickness of the tile in the approximate center of the face 12. A countersunk region 34 typically exists within the hole or bore 30 to act as a seat for a nut 28. In a typical installation, a stud or threaded bolt 24 is welded to a membrane region 16 between two tubes 18 in the boiler wall 62. The contoured back surface 14 of the tile 10 includes accurate portions 20 sized and shaped to receivably match the profile of the tube wall 19 and allow for close contact therewith.
The tile is installed by fitting the stud through the hole and securing the tile in position with a washer 28 and nut 26 threaded onto the stud. A cap (not shown) is then typically mortared in place over the hole to minimize the amount of gas that can flow through clearance between the standard hole to the backside of the tile. A layer of mortar (not shown) is typically applied between the tile and the wall 62, and between adjacent tiles, to form a rigid structure which serves to help secure the tile in position and to substantially prevent gas from flowing between and/or behind the tiles.
An advantage of using a bolted tile configuration is that the tiles are relatively easy to install and may be installed on substantially any surface of the boiler, including vertical and overhanging surfaces.
A disadvantage of such bolted tiles is associated with tile failures. These tiles usually have a 2 to 4 year life expectancy due to stud failure. When the studs fail, the tiles tend to fall off of the walls, leaving the boiler tubes exposed to the incinerator atmosphere. The cause of failure in the studs was previously believed to have been due to high temperature acid corrosion. In particular, it was believed that the acids penetrated the tile through the stud hole to attack the stud. The corrosion was believed to be severe on the stud due to its high operating temperature (believed to be 1000.degree. C. or more). Also, cracking of the tiles was a common occurrence, and believed to have been generated by overstressing the stud.
One attempt to address the disadvantages associated with bolted tile systems has included the use of hanging tiles. As shown in FIGS. 3 and 4, hanging tiles 45 typically utilize an anchor/hook or short stud 37 to hang the tile on the membrane 16 of the boiler wall 62, with gravity utilized to maintain the tile in close proximity to the boiler wall 62 for good heat transfer. This tile 45 is a modification of the bolted tile in that one or more holes 30' project from the back surface 14' of the tile toward the hot face 12, but do not fully penetrate the hot face. This provides the tile with a closed face, to theoretically improve acid corrosion resistance. A layer of mortar (not shown) is typically installed between the tile 45 and the boiler wall 62, as well as between adjacent tiles to form a rigid, substantially gas and ash impermeable structure.
A variation of this hanging tile arrangement utilizes the tiles 45 in conjunction with an air sweep system. In this variation, no mortar is installed behind the tiles to leave a gap between the tiles and the boiler wall. A flow of air is fed through this gap to help minimize acid corrosion of the wall.
An advantage of hanging tiles in general is the relative ease of installation and replacement. A disadvantage of such hanging tile arrangements is that the tiles generally cannot be installed on non-vertical walls, as the tiles tend to fall off their anchors. Also, tiles have been known to lift off of their anchors during operation due to thermal expansion, etc. Moreover, the above described air sweep system tends to disadvantageously increase the expense of the tile system relative to configurations utilizing mortar. Heat transfer between the tiles and wall also may be disadvantageously reduced due to the insulative (i.e., relatively low thermal conductivity) characteristic of air layers.
Turning to FIG. 5, modified hanging tiles 50 have been developed in an attempt to address the drawbacks associated with the hanging tiles becoming dislodged from their anchors or hooks. Examples of such modified configurations are commonly known as mushroom bolt, tube-welded fin anchor, and T-slot tiles. These tiles 50 are typically hybrids of bolted tiles and hanging tiles, incorporating a closed tile face 12' with an anchor 52 that has a substantially T-shaped profile, to effectively capture the tiles and allow them to be installed on both vertical and overhanging surfaces.
These tiles 50 are generally installed with a layer of mortar between the tile and the boiler wall, as well as between adjacent tiles. The purpose of the mortar is to help secure the tiles by providing a rigid attachment and to provide a barrier to resist penetration of ash and corrosive gas between and behind the tiles.
An advantage of these modified tiles 50 is that they may be installed on nominally any boiler surface. Disadvantages of the tiles 50 include difficulty of manufacture since they incorporate a blind (i.e., discontinuous) slot 54 projecting laterally into the tile from an edge thereof. Also, the tiles may be physically weaker due to the complexity of the blind slots 54. Moreover, individual tiles generally cannot be replaced without removing a entire row of tiles.
An additional approach intended to address the drawbacks of the above-described configurations has included use of the hanging tiles 45 in combination with a resilient material 48 installed between opposed recesses or grooves 49 as shown in FIGS. 3 and 4. This approach tends to facilitate installation and replacement of the tiles relative to tile systems utilizing conventional rigid mortar. While this approach may operate satisfactorily in some applications, the resilient material 48 disadvantageously provides little resistance to corrosive gas flow and thus tends to be undesirable for use in particularly corrosive environments such as found in MSW boilers.
Still further, many of the approaches discussed hereinabove utilize a fibrous compressible material or mortar at periodic tile intervals to serve as expansion joints. For example, such material may be used every 7 to 15 tiles in a manner familiar to those skilled in the art of masonry (i.e., such as commonly utilized in fabrication of concrete sidewalks, etc.) These joints however, tend to disadvantageously permit the passage of acids and other corrosive materials therethrough, to enable corrosion of the underlying boiler wall. Additionally, the useful life of tile systems having such expansion joints have not been shown to be appreciably greater than similar tile configurations not having such expansion joints.
Thus, a need exists for an improved refractory tile system that addresses drawbacks associated with the prior art.